The Mirror Dimension is a fictional dimension in the Army of Darkness / Evil Dead universe which first appeared in 1987 in the movie "Evil Dead II", though it wasn't identified as the Mirror Dimension at that time.
The Mirror Dimension is where the first Chosen One banished the Dark Ones and Deadites. The spirits of slain demons and Deadites live in this dimension until someone summons them to Earth, alongside the souls of those consumed by the Deadites. It's unknown as to who created the dimension. Entry to the dimension is via a portal created by a spell in the Necronomicon, a Deadite swallowing your soul, or being pulled into it by a doppelganger via a reflective surface.
One can leave dimension by two ways, a teleportation spell cast by a servant of the Dark Ones or if you were pulled in by the doppelganger, by returning to the same spot and switching places. It's implied that the dimension is somewhat sentient. Changing its appearance, using the "swallowed" souls that joined the group consciousness as inspiration. Those who are pulled into the dimension experience death over and over, in varying ways (e.g. melting, falling apart, being consumed / replaced by worms, etc). When a simulacrum of Ash Williams ended up in that dimension, it experienced or witnessed its death a million times.